Lennox Lewis thought former promoter Frank Maloney's decision to undergo a sex change was a "wind-up".
Sixty-one-year-old Maloney, who guided Lewis to the world heavyweight title, has told how she now lives as a woman called Kellie.
Maloney wants the boxing community to be open minded about his situation - and Lewis respects the decision.
Lewis, now 48, issued a statement on Facebook which read: "I was just as shocked as anyone at the news about my former promoter and my initial thought was that it was a wind-up.
"The great thing about life, and boxing, is that, day to day, you never know what to expect. This world we live in isn't always cut and dried or black and white, and coming from the boxing fraternity, I can only imagine what a difficult decision this must be for Kellie.
"However having taken some time to read Kellie's statements, I understand better what she, and others in similar situations are going through. I think that all people should be allowed to live their lives in a way that brings them harmony and inner peace.
"I respect Kellie's decision and say that if this is what brings about true happiness in her life, than so be it. #LiveAndLetLive."
Maloney called time on a 30-year career in boxing last October.